Mechanical stop for potentiometers



Feb. 3, 1959 HATHELD 2,872,553

MECHANICAL STOP FOR POTENTIOMETERS Filed Sept. 19, 1955 BY M 29 igfy IINVENTOR:

United States Patent MECHANICAL STOP FOR POTENTIOMETERS Lester N.Hatfield, Flushing, N. Y., assignor to Fairchild Camera and InstrumentCorporation, a corporation of Delaware Application September 19, 1955,Serial No. 535,195 1 Claim. (Cl. 201-48) This invention relates toelectrical resistors such as potentiometers, and more particularly tomechanical stop arrangements for potentiometers.

The use of precision potentiometers of all types has become verywidespread in many different classes of electronic equipment, with theresult that much effort has been expended to reduce the cost ofprecision potentiometers by mass production methods. Since variousapplications often require potentiometers which have rotary wipercontacts travelling through different total arcs, although otherwisesimilar in construction, it is desirable to be able to use the samemass-produced resistance element in all such devices, the regulation ofthe travel are being set as desired for each potentiometer. It is hencea principal object of this invention to provide a mechanical stoparrangement for potentiometers that will permit the use of the same typeof resistive element for potentiometers requiring different arcs oftravel of the wiper or contact element.

In the conventional rotary potentiometer, the mechanical stop utilizedusually consists of a radial pin secured to the wiper or contact shaft,and a stop pin located on an inner portion of the potentiometer frontplate, rear plate, or housing. Thus the total are of travel iscontrolled by the diameter of the radial pin and the diameter of thefixed stop pin and its radial location from the center of the shaft.When the total angle of the arc approaches 360, the diameter of bothpins must necessarily be decreased, assuming a fixed radial location.Such a reduction in the pin size will result in a reduction of theamount of torque that can be applied to the wiper or contact shaftwithout rendering the stop ineffective. It is hence another object ofthis invention to provide a potentiometer stop that is capable of usewhere the total are of travel approaches or even exceeds 360, and can besubjected to large torque values without rendering the stop ineffective,or injuring the same.

It is another object of this invention to provide a potentiometer stoparrangement that will permit the use of the same potentiometer fordifferent arcs of wiper contact travel merely by changing a singleeasily accessible element.

Still another object of this invention is to provide a potentiometerstop mechanism which includes an interchangeable floating plate todetermine the arc of travel of the contact element with high accuracyand precision.

The above as well as other objects and advantages of V the inventionwill be more readily understood from the following detailed drawings, inwhich Fig. 1 is an explodedperspective view of one embodiment of thestop mechanism.

Fig. 2 is a vertical cross sectional view of the device in Fig. 1,except that all the elements are shown in their actual operatingrelationship.

Fig. 3 is a perspective view of a modified form of floating plate thatmay be used in the mechanism illustrated in Fig. 1.

2,872,553 Patented Feb. 3, 1959 Fig. 4 is a plan view of the floatingplate illustrated in Fig. 3.

Fig. 5 is a perspective view, partially broken away, of a furthermodification of the floating plate of Fig. 1.

Fig. 6 is a view similar to Fig. 1 showing a modified arrangement.

Fig. 7 is a schematic side elevation of a further modification.

Referring to Figs. 1 and 2, reference numeral 10 desig nates a frontarea bearing plate provided with a circular opening 12 for an actuatoror contact shaft 14. Mounted on the end of the actuator shaft 14 is awiper or contact blade 16 having a hub 18. The Wiper blade 16 is securedto the shaft 14 by means of a press fit on the hub 18 by set screws orby any other suitable means.

Front plate It) will normally constitute the front plate of acylindrical potentiometer casing within which the resistance element,wiper arm 16 and other parts will be mounted. Since the presentinvention is not concerned with the resistor element itself, or thecasing, these parts are shown diagrammatically, the resistor beingindicated at R and the casing at C in Fig. 1.

Situated below the wiper blade 16 and also rigidly mounted on the shaft14 is a circular shaft stop plate 20 provided with a center hole to passthe shaft 14. The underside of the shaft stop plate 20 is provided withan arcuate slot 22 of rectangular cross section. For the purpose ofengagement with a stop member to be described in more detail below,there is provided a slender bar, pin or strut 24 bridging the arcuategroove 22 on one side only of the shaft stop plate 20; i. e., at asingle radial location in the groove.

Between the shaft stop plate 20 and the front plate 10 there is situateda floating plate 26. The floating plate 26 may be in the form of a thinmetal disc having a center opening for receiving the shaft 14, thecenter opening being sufliciently large to permit the plate 26 to rotatefreely about the shaft 14, and its position along the shaft 14determined in relation to pin 24 and pin 32 by means of spacers or othersecuring devices. The floating plate 26 is provided near its peripherywith stop pins 28 and 29 which project toward the shaft stop plate 20and which, when the parts are assembled in operative relationship, enterthe arcuate slot 22 in order to engage pin or strut 24 upon relativerotation between the shaft stop plate 22'} and the floating plate 26.Also situated near the periphery of the floating plate 26 and extendingin the direction of the front plate 10 is a third stop pin 30. Therelative angular placement of stop pins 28, 29 and 30 is determined bythe maximum extent of are it is desired to move the wiper or contactblade 16.

As has been said, the stop pins 28 and 29 are meant to engage the pin orstrut 24 bridging the arcuate slot 22. The depending stop pin 30 ismounted for cooperation and contact with a further stop pin or lug 32mounted fixedly upon the front area bearing support 10.

The assembly and operative relationship of the parts is clearlydisclosed in Figs. 1 and 2. The floating plate 26 is so positioned thatupon rotation of same stop pin 30 will engage stop pin or lug 32. Therelative positioning of floating plate 26 and shaft stop plate 20 issuch that stop pins 28 and 29 enter the arcuate slot 22 and engage pinor strut 24 upon relative rotation of the plates 20 and 26. As may beseen in Fig. 1, pin or strut 24 is free to traverse the angle 31 uponsuch relative rotation. The shaft stop plate 20 is secured to theactuator shaft 14 and of course is so positioned that the arcuate slot22 faces the front plate 10. It will be understood that the shaft 14 ismounted for rotational movement upont front plate 10, and thatpositioning washers or spacing means may be included between theelements of the assembly, if desired.

The operation of-themechanism is simple and straightforward. When theactuator shaft 14 and shaft stop plate are rotated in acounter-clockwise direction (as viewed in Fig. 1), strut 24 engages pin28 and thereby causes floating plate 26 to rotate in a counter-clockwisedirection until pin 36 strikes pin or stud 32. The shaft 14 is then atone limit of its travel. When the shaft 14 is rotated in the other orclockwise direction (as viewed in Fig. 1), plate 20 rotates with respectto floating plate 26 until strut 24 engages pin 29, the relativerotation between the two plates equalling the angle 31 between the pins23 and 29 (approximately 60 as shown in Fig. 1). Upon the engagement ofstrut 24 with pin 29, the floating plate 26 is rotated until pin 30engages pin or stud 32 as a result of clockwise rotation, the shaft 14then being at the other limit of its travel. It will be seen that therelative rotation between floating plate 26 and front plate 10 amountsto 360 less the lateral thickness of the pins 30 and 32. As statedabove, the relative rotation between floating plate 26 and shaft stopplate 2% equals the angle 31 between pins 28 and 29. It thus becomesapparent that the shaft 1 is allowed to rotate through an angle greaterthan 360, namely the summation of the relative rotational displacementsbetween the plates 2! and 26 and plates 26 and 10.

A greater rotational displacement between the shaft 14 and front plate10 may be achieved with the structure of Fig. l by merely repositioningthe floating plate 26 so that the strut 24 traverses the complementaryportion of the periphery of plate 26, that is, the larger angle betweenthe pins 28 and 29, instead of the smaller angle 31. If the pins 28 and29 are placed closer together, with strut 24 positioned so as totraverse the larger angle between same, the obvious result is that ofgreater rotational displacement. The limit of such alteration is reachedwhen the two pins 28 and 29 coincide, that is, when there is but onesuch pin on the upper face of floating plate 26. In this limitingposition, the maximum rotation of shaft 14 Withrespect to front plate 10is almost 720.

If an even greater displacement is desired, as may be in certain servoapplications, additional floating plates may be included in the assemblyin such manner that each of the plurality of floating plates is inoperative (pinengaging) connection with an adjoining plate, therebyaffording approximately 720 of additional displacement per plate.

Within the limits of variation afforded by each floating plate that maybe utilized, the precise degree of rotation allowed may be set by theangular displacement between the stop pins of such plate. Adjustment ofthe relative angular positioning of stop pins may be accomplished by theuse of movable stop pins or alternatively, and more desirably from thestandpoint of practicability, such adjustment may be accomplished by thereplacement of a given floating plate by another floating plate ofsimilar functional characteristic but whose stop pins are positioned ata different relative angular displacement. In this manner, changes inthe maximum permissible rotation of the potentiometer shaft may beaccomplished by merely interchanging floating plates.

An alternative form of floating plate is illustrated in Figs. 3 and 4.In this embodiment, the floating plate 34 is provided with stop members36, 33, 4t and 42 in the form of tabs which have beenstamped or punchedfrom the disc itself. This type of construction is simple and may beproduced economicallyby a metal stamping process or the like, and thusthe limits of angular movement in production items may be readilycontrolled as to accuracy without-tedious and expensive manualadjustment. The stopmembers 36 and 38 are adapted to cooperate with pinor strut 24 in a manner similar to the relation between-the stop pins 28and 2? and said strut, and stop members 40 and 42 cooperate with stoppin 32 in a manner similar to that described in connection with stoppins 30 and '32. It will be understood that the ,stop

members 36 and 38 define the limits of travel ofpin or strut 24 withrespect to floating plate 34, and stop members 40 and 42 similarlydefine the limits of travel of floating plate 34 with respect to stoppin 32. As in the case of the previously described embodiment,additional relative rotational displacement may be had between shaftstop plate 2}) and front plate 16 by the inclusion of additionalfloating plates in the assembly. Conversely, this displacement may berestricted by placing closer together the stop members 36 and'38 or thestop members 40 and 42, or both.

As will be obvious to those skilled in the art, the primary reason forthe inclusion of a pair of stop members of the stamped tab type in placeof the single stop pins of Figs. 1 and 2 is due to the fact that thestamped tabs offer but one positive'and accurately defined activesurface. In order to obtain accuracy of operation, it is thereforenecessary in this latter embodiment to provide a pair of oppositelydirected stop members on both sides of the floating plate 34.

Fig. 5 shows a further modification of the floating plate of Fig. l,which modification is similar to that of Figs. 3 and 4 except that thestop member 46 of the floating plate 44 in Fig. 5 is struck up in suchganner as to present an accurately positioned stopping surface on eitherside of the stop member. in the use of a floating plate of this type thestrut 24 of the shaft stop plate 26 engages either side of stop member46 in a positive fashion, similar to the action of the stop pins 23 and29 of Fig. 1. Thus it is not necessary to provide pairs of stop membersas needed in the modification of Figs. 3 and 4, since there are nolonger any cam-like surfaces which may be presented to the cooperatingstrut 2 5. Obviously, this type of struck-up tab or stop member mayequally well be used in cooperation with the stop pin or lug 32 of Fig.1.

Fig. 6 of the drawings shows a further modification in which the plate2% is replaced by a simple radial pin 24 secured directly to shaft 14 asby being pressed into a suitable radial hole in the shaft. Obviously,such a pin 24 could equally well be secured to a sleeve or bushing whichis in turn fastened to the shaft 14 as by set screws or the like. InFig. 6, only the parts of Fig. l necessary to show this modification areincluded, as it will be understood that the casing and resistor elementwill occupy the same relative positions as in the former figure.

it will also be obvious to those skilled in the art that while Fig. 1shows the pin 2 and floating plate 26 arranged between the wiper arm 16and the bearing plate or cover it), it is equally feasible to arrangethe pin 24 and plate 26 between the wiper arm and the casing C. In thiscase, as shown schematically in Fig. 7, the stop element 32' would besecured to the underside of the end of easing C, rather than to thecover plate iii.

While the invention has been described herein in connection with certainpreferred embodiments and procedures, it is to be understood that theseare given by way of illustration and example and that the inventionitself is not limited thereto except insofar as required by the scope ofthe appended claim.

What is claimed is:

In a potentiometer of the type having an arcuate resistive element forcontact by a wiper arm, the combination comprising a front bearingsupport having an opening therethrough, an actuator shaft positioned forrotation within said opening, a wiper arm mounted on said actuatorshaft, a shaft stop plate between said wiper arm and said front bearingsupport, said shaft stop plate mounted on said actuator shaft forrotation with said shaft, 21 floating plate between said shaft stopplate and said front bearing support, said floating plate looselymounted on said actuator shaft to permit free rotation of said shaftwith respect thereto and having a pair of 5 6 spaced independent stopelements projecting from one References Cited in the file of this patentfi f F 2 21 igs? and adsiligla 2 g: UNETED STATES PATENTS en pro1ec1n re18 o ace, an sop m m i on said stop plate and said front bearing supportfor co- 01,710 g fi g g acting respectively with the stop elementsprojecting 5 2156'O67 i i 3 from the opposite faces of said disc.2591650 W1 lams FOREIGN PATENTS

